Plastic media filter

ABSTRACT

A filter for removal of biochemical oxygen demand and suspended solids utilizing multi-layer filtration media. The filter media includes plastic particles, sand, and garnet and is operated under an automatic control condition. Fine filtering is accomplished by a sludge cake which develops at the upstream face of the filter media. For cleaning the filtration media, deposited sludge is removed from the filter bed by backwashing in a short time period at a high strength backwash flow rate.

This is a continuation-in-part of U.S. Pat. application Ser. No.08/024,433 filed Mar. 1, 1993, now U.S. Pat. No. 5,350,505.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention generally relates to filter systems for thetreatment of wastewater and potable water. In particular, the filtersystem of the present invention comprises a vessel containing multiplefilter media situated in layers. The system utilizes a downflow filtertechnique including a periodic backflow process.

II. Background and Description of the Related Art

Home treatment plants are extensively used to treat wastewaterdischarged from houses which are far from city sewer systems. In fact,each minute, 4.2 million gallons of wastewater are discharged from hometreatment plants in the United States alone. The effluent quality ofwastewater from home treatment plants is evaluated using fourparameters: dissolved oxygen ("DO"), pH, biochemical oxygen demand("BOD") and suspended solids ("SS"). Treated wastewater must meetcertain required acceptable tolerance ranges which are set forth intypical secondary effluent standards such as the Class I dischargestandard of National Sanitation Foundation ("NSF") International.Usually, pH and DO requirements are easily achieved, and an upflow ordownflow gravel filter is used as the last process to remove BOD and SSin some home treatment plants. However, many gravel filters are onlybackwashed after several months of service. Absent such backwashing,medial clogging problems can occur, resulting in inconsistent effluentquality. Similar filtering methods are used to treat effluent fromcommercial plants. However, poor maintenance of such systems alsoresults in similar problems, leading to poor effluent quality.

In wastewater treatment applications in particular, the presence ofsuspended solids is frequently a major process problem. Filtration hascommonly been employed to remove suspended solids from wastewater.Filtration normally occurs as the third step in a tertiary treatmentprocess, following processing in a settling tank and a biofilmdegradation process. Although traditional sand and mixed media filtersare generally effective in removing suspended solids, the filtration bedis susceptible to clogging and high pressure drops across the bed. As aresult, sand and mixed media filters require a manually controlledbackwash procedure to clear accumulated solids.

Based upon the above problems, new types of filters have been developed,such as the filtration bag, the filtration sock, and polyurethane mediaand synthetic media filters. However, the filtration bag and sockrequire periodic replacement. Further, other filters are not suitablefor home plants, which operate automatically and are inspected only onceevery six months. An invention that addresses the clogging problem isdescribed in U.S. Pat. No. 4,906,381 to Barbaro, which discloses a fluidfiltration unit comprising a number of filter modules. Each module has apressure release valve. When a particular module becomes clogged withsolids filtered from the fluid, the release valve allows that module tobe bypassed and the fluid passes to the next unclogged module.

Recently, granular plastic media have been used as filtration media, butsome types of plastic media are used only under gravity backwashconditions. If these media are used in a pump backwash system, theplastic media is coagulated into large particles by electrostatic force.This coagulation adversely affects backwash and filtration efficiencies.

Past attempts at filtering suspended solids in sand filters involvedsome manner of arranging the orientation of filtration media by placingthe finer sand on the bottom and the larger grained sand on the top.This arrangement is contrary to that of a rapid sand filter, in whichthe finer grained sand is layered atop the larger grained sand. By usingthis arrangement in a downflow filter, much of the filter cloggingexperienced with the rapid sand filter can be prevented. The larger sandtraps large solids, allowing finer particulate matter through to befiltered by the fine sand. The problem with such an arrangement has beenthat of maintaining the orientation of fine sand at the bottom and largesand at the top during backwashing. Upon backwashing, the larger sandparticles tend to settle to the bottom so that resulting orientation isalmost equivalent to the orientation of an ordinary rapid sand filter,that is, the larger grained sand settles to the bottom and the finersand to the top. In practical application, anthracite particles and sandare used in double layer media filters because the specific gravity ofthe anthracite particles is less than that of the sand although theanthracite particles are larger. The anthracite particles thereforesettle atop the sand after backwashing, and the desired media layeringis maintained. The structure of the anthracite particles is too fragileto last through a long operation period, however. The problem of losingthe anthracite particle media during backwashing is a significantdisadvantage of this process.

Some of the above problems have been addressed by other inventions. Forexample, U.S. Pat. No. 3,814,247 to Hirs discloses a filter system usingtwo granular filter media layers. The top media layer is composed of acourse grained material. The layer beneath this is composed of a finergrained material having a specific gravity that is greater than that ofthe material above it. Synthetic as well as natural materials aredisclosed as possible filter media for the upper layer. Backwashing ofthe lower layer is accomplished in the usual manner whereas the upperlayer undergoes a more violent agitation and slurrying in a flow pathexternal to the filter vessel. Frequent backwashing in this manner isperformed in order to avoid a buildup of suspended solids at the face ofthe upper filtration medium.

U.S. Pat. No. 4,197,205 to Hirs discloses a filter system using a numberof granular filter media layers. The top media layer is composed of acourse grained material. The layers beneath this are composed ofprogressively finer grained materials having specific gravities thatbecome progressively greater than that of the material above it. Atleast one layer is composed of a synthetic material. The layers arearranged in this fashion in order to reduce the occurrence of surfaceplugging.

U.S. Pat. No. 4,246,119 to Alldredge discloses an upflow or combinationupflow/downflow filter system using a number of filter media layers. Thefilter media are compressed between two flexible diaphragms, one locatedabove the filter media and one below. In the upflow configuration, finerfilter media are layered on courser media. In the combinationupflow/downflow configuration, a fine grained filter medium is packedbetween two courser media layers.

U.S. Pat. No. 4,692,248 to Stannard et al. discloses a filter apparatuswhich filters material from influent through a solid filter medium. Afilter cake which forms on the surface of the solid filter medium iscontinuously washed away in the disclosed process utilizing theapparatus.

U.S. Pat. No. 4,851,122 to Stanley discloses a filter apparatusutilizing five layers of filtration media. The water to be treated firstencounters an activated charcoal layer, used as a bacterial-reducingagent. Three successive layers of resin based media then removeimpurities from the water. Finally, a layer of filtration medium such ascrushed quartz, having a particle size much greater than that of theresin layers, removes sediment from the water.

None of the above inventions addresses nor solves the problems of filterefficiency or head loss over long periods of use. Head loss is theheight difference between the water level in a fluid treatment plant andthe treatment plant outlet. Head loss during the filtration cycle shouldbe minimized for efficient water treatment. Initially, head loss duringa filtration process is caused by the resistance of the filtration mediathemselves. As the media become clogged with impurities, it is thisresulting sludge which increases the head loss of the system.Backwashing alleviates the head loss problem, but inefficientbackwashing and clogged filter media following the backwash processincrease head loss over the course of the wastewater treatmentoperation.

Those inventions which use synthetic filter media still have the problemof media coagulation following backwash. Loss of natural media throughlong term use and-vigorous backwash continues to be a problem. Thedisclosed processes are not suited to automatic operation, especiallyoperation including an effective backwash procedure, nor are the systemseasily maintainable.

Faced with the foregoing difficulties in the application of conventionalfilters and new types of filters, the new multi-media filter of thepresent invention has been developed to improve filtration efficiencyand head loss across the media, and eliminate the media loss problem.

It is therefore an object of the present invention to provide a filterfor tertiary treatment of municipal wastewater which possesses theadvantages of high filtration efficiency and easy maintenance.

It is a further object of the present invention to set up a media layerarrangement which has the largest grained filter medium on the top andthe finest grained filter medium on the bottom, exactly the opposite ofthe rapid sand orientation.

It is another object of the present invention to use anti-electrostaticgranular plastic media, which has a long practical life and does notaffect filtration and backwash efficiency.

It is a further object of the present invention to provide an automaticfiltration and backwash control system which does not need any manualcontrol at all, and is inexpensive and simple to use.

It is yet another object of the present invention to provide a filterwhich will achieve the Class I discharge standard of NSF International.

SUMMARY OF THE INVENTION

The present invention comprises a holding tank packed with triple-layerfilter media. The influent containing suspended solids is flowed througha check valve, top screen, and filter bed, which consists ofanti-electrostatic plastic media, sand, and garnet. The filtered fluidis then flowed through a bottom screen, a collection and distributionpipe, and a backwash pump. Mechanical straining occurs when the sludgeparticles in the influent are larger than the pores of the filtrationmedia. At the beginning of the filtration period, large particles andbulked activated sludge are removed at the top of the plastic medialayer and fine particles of activated sludge are removed at the top ofthe sand layer. Straining occurs at the top of the plastic media layerafter a "sludge cake" develop on the top of the filtration bed. Thiscake consists entirely of particles strained from the influent.Consequently, the pores in the cake are generally smaller than theparticles in the influent which formed it. New particles arecontinuously added to the upstream face of the sludge cake, and the cakegradually thickens. Particle removal is now accomplished by the cakeitself. The original filtration bed merely acts as a support for thecake. The cake removes particles from the influent by mechanicalstraining and flocculation at its upstream face.

The treatment provided by the filter is not only mechanical strainingand physical entrapment, but also biofilm degradation. Usually, theinfluent is aerobic, containing certain concentrations of DO andbiomass, and its effect in the filter is to promote the growth ofaerobic bacterial film on the surface of the filter media. BOD in theflow is removed, accompanying SS removal and the biofilm degradationprocess.

Because the anti-electrostatic plastic particles are not worn out in ashort period, the filtration head loss is low and sludge removalcapacity is high. The plastic media can also be fluidized easily duringbackwashing. Unlike some currently used filters which utilize anthraciteas a filter medium, the present invention utilizes plastic particles,which are man-made and therefore can be manufactured to a uniform size.Further, these particles do not wear out as do anthracite particles,which occur naturally and therefore do not have an optimal uniformitycoefficient.

During backwashing, backwash fluid in the effluent holding tank ispumped into the filter through the backwash pump, collection anddistribution pipe, bottom screen, filtration bed, top screen andbackwash outlet. The check valve on the influent line is closed byhydraulic force, and the backwash effluent is forced to flow out througha backwash effluent line. The media undergoes fluidization between topand bottom stainless steel screens during this backwash process. Thesludge cake and particles accumulated in the filtration bed are washedout from the filter in a short time. After backwashing, the media settledown in order, and the resulting syphon phenomenon is broken by a holein the effluent line, which always assures the media remains submergedin the liquid. The filter bed is then reset and the filtration processis resumed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the filter assembly in operationshowing fluid flow.

FIG. 2 is a schematic diagram of the filter assembly during thebackwashing process.

FIG. 3 is a schematic view of a tri-media filter in operation, such asmay suitably be used in the practice of the present invention.

FIG. 4 is a schematic view of a tri-media filter during the backwashingprocess, such as may suitably be used in the practice of the presentinvention.

FIG. 5 is a plan view of screens employed in filters of this invention.

FIG. 6 is a schematic view of a dual-media filter in operation, such asmay suitably be used in the practice of the present invention.

FIG. 7 is a schematic view of a tri-media filter in operation, with thebottom screen replaced by a filtration brick

FIG. 8 is a schematic view of a tri-media filter in operation, after asludge cake has formed at the top of the filtration media.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the filtration system is shown. Influent liquidcontaining certain concentrations of BOD and SS enters the filtrationsystem via influent line 1 having check valve 2, which is open duringthe filtration phase operation. This influent liquid may be derived froma variety of sources. For example, a biological treatment plant effluenthaving 35-80 parts per million SS or river water for a municipal potablewater treatment facility may be inputs to the system. Alternatively, theliquid could comprise raw water intended for boilers or for internalindustrial plant use. From influent line 1, the liquid enters a closedcontainer, filter vessel 3. The filter vessel 3 is located within alarger closed container, referred to as the backwash water holding tank13. Alternatively, the filter vessel 3 may be placed beside the backwashwater holding tank 13 rather than inside it, or the two may be placedremote from each other. They may then be connected by a hole, pipe,channel, or other effluent line. The filter vessel 3 may suitably beconstructed from any rigid nonporous material. Suitable materials forconstruction of the filter vessel 3 include PVC or polyethylene or othertypes of plastic structure material, as well as steel and concrete. Thefilter vessel 3 is only partially filled with filtration media. Thefilter vessel 3 has an influent inlet and backwash outlet at the top,and a collection and distribution outlet at the bottom. One stainlesssteel screen is installed at the top to prevent media loss duringbackwash, and another screen at the bottom to support the filtrationmedia.

In the preferred embodiment, the hydraulic loading of the filter isdesirably maintained between 1 and 6 gal./min./ft.² and the filtrationperiod lasts between 3 and 24 hours, the exact values to be determinedby the amount of hydraulic loading, influent SS loading andfiltration-allowed head loss. The effluent from the filter passesthrough the backwash water holding tank 13. The effluent rises to thelevel of an outlet port, tank outlet which channels the overfloweffluent to receiving waters of other treatment facilities, or elsewhereto be made available for other uses. The amount of water that remains inthe backwash water holding tank 13 is available for use during thebackwash process, and occupies the net effective volume of the backwashwater holding tank 13, which is between 15 and 60 gallons.

Another option available is to introduce a lift station next to thebackwash water holding tank 13. The lift station tank may be designed asa separate tank or as a combination tank including the backwash watertank. The lift station may be necessary to pump effluent from thebackwash water holding tank 13 when the filter is located below groundlevel or below the level of the receiving waters of subsequent treatmentfacilities.

During the backwash step, the liquid in the backwash water holding tank13 is pumped into filter vessel 3 by backwash pump 12 See FIG. 2. In thebackwash step, the check valve 2 is closed by backwash water pressure,and the backwash effluent is flowed through backwash effluent line 15 toa treatment facility such as a primary settling tank or an aerationtank. In larger filters such as those used in commercial applications,the backwash effluent line 15 may be a large effluent open channel. Thebackwash process may be controlled automatically by either a cycle timer16 or a float switch 20.

After the backwash step is terminated, the backwash water remaining inthe filter flows out effluent line 10 by gravity flow force which causesa syphon phenomenon. The syphon phenomenon is broken by an anti-syphonhole 11, which provides a passage for fluid between the inside of theeffluent line 10 to the backwash water holding tank 13. The water linein the filter vessel 3 is kept at the same level as the anti-syphon hole11. The filtration media is always kept submerged under liquid by usingthe anti-syphon hole 11 to break the syphon phenomenon. If theanti-syphon hole 11 were not present, the liquid would all be drawn outof the filter vessel 3 following the backwash procedure, and the filtermedia would be dried, which would adversely affect the filtrationefficiency. Once the level of the liquid in the filter vessel is loweredby gravity flow, the check valve 2 is automatically opened by theinfluent hydraulic pressure. Now the filter can begin a new filtrationportion of the cycle.

The backwash water added to the filtration vessel 3 in the backwash stepmay contain suspended solids which consist of fine sludge particlespassed through the filtration media and flocculated in the backwashwater holding tank 13. The solids are introduced to the filter vessel 3at the beginning of the backwash step. Even though there are some solidsstill remaining in the backwash water holding tank 13 after filtration,filter performance and filtrate quality are not affected.

As stated earlier herein, the backwash water volume is between 15 and 60gallons, the exact volume determined by the treatment facilitiessupplying influent to the filter system and by the backwash effluentreceiving facility. For example, if the backwash effluent is introducedto a primary settling tank, a small volume (15 to 25 gallons, or 2-4percent of daily effluent) of backwash water must be employed to preventhydraulic shock loading to the treatment system.

The filter vessel 3 will now be described in detail. During thefiltration step, as shown in FIG. 3, the liquid containing SS and BOD isflowed through the top screen 4, anti-electrostatic granular plasticmedia layer 5, sand layer 6, garnet layer 7 (optional), bottom screen 8,and collection and distribution pipe 9, before passing on to thepreviously described effluent line 10 having a small hole 11 in the toppipe and backwash pump 12 and being discharged through the tank outlet14. As previously noted, the bottom screen may be replaced by afiltration brick 22, as shown in FIG. 7. The granules of the plasticmedia layer 5 are larger in dimension than are the grains composing thesand layer 6. As a result, the large size sludge particles are removedat the top of the granular plastic media layer 5, the medium size sludgeparticles are accumulated in the granular plastic media layer 5, and thesmall size sludge particles are strained at the top of the sand layer 6and the garnet layer 7. The effluent from the filter is passed toreceiving waters of other treatment facilities, or is made available forother uses.

As shown in FIG. 3, the garnet layer 7 is actually two layers of garnetin the preferred embodiment. The lower layer of garnet is composed oflarge sized garnet pieces, the upper layer of smaller sized garnetparticles. The lower layer rests directly on the bottom screen 8 andsupports the other filter media. The bottom screen 8 functions as asupport for all the filter media. The larger sized garnet particles willnot impede fluid flow through the bottom screen 8.

The previously described filtration process is continued until a "sludgecake" 24 is formed at the top of granular plastic media layer 5, asshown in FIG. 8. After this time, most SS particles in the influentliquid are strained at the top of the cake, whereupon the filtrationhead loss is gradually raised and the filtration bed is ready forbackwashing.

To ensure that the filter media utilized in the present invention returnto their original orientation after the backwash process, the granulescomposing the plastic media layer 5 have a specific gravity that islower than that of the smaller grains of the sand layer 6. Likewise, thespecific gravity of the pieces making up the garnet layer 7 is higherthan that of the grains in the sand layer 6. Thus, the layers settle inthe proper order after backwash, that is, garnet layer 7 on the bottom,sand layer 6 in the middle, and plastic media layer 5 on top. In thepreferred embodiment, in which the garnet layer 7 is composed of twodifferent sizes of garnet pieces, the smaller sized garnet pieces settleatop the larger sized garnet pieces.

It has been found that many different types of plastic particles haveelectrostatic properties during backwash which affect backwash and mediarenewal efficiencies and filter operation life. The anti-electrostaticgranular particles used in the filtration bed of the present inventionmay be any suitable granular plastic particles which do not experienceelectrostatic attraction during a pump backwash. Examples of suchanti-electrostatic plastic media include 33% glass filled nylon 6/6granular particles manufactured by Entec Polymers, Inc. Such particlesmay be weighted differently in order to achieve the desired specificgravity.

The anti-electrostatic granular plastic particles employed in thepresent invention have a uniform particle size. This quality isimportant to avoid having finer plastic granules rise to the top of thegranular plastic media layer 5 after backwashing, reducing thefiltration effectiveness of this layer. The shape of the particles maybe spherical, cube shaped, shaped in a pillar form, pelletized, or anyshape that may be obtained by physically cutting a body of plastic wireinto small size particles. Suitable glass-filled nylon pellets whichhave been tested successfully have a specific gravity of 1.3, less thanthat of the sand, which in turn has a lower specific gravity than thegarnet. The combination of uniform size and shape chosen must allow theplastic particles to be suitably strong, that is, to have a suitablehigh capacity for solids loading and allow for low filtration head lossduring the filtration step. The size of the media used is alsodetermined by the effluent quality limits desired for the filter.Smaller media will filter more SS. The depth of each media layer ischosen based on the desired fluid flow rate; deeper media layers willslow the flow rate.

During the backwash process, as shown in FIG. 4, the liquid is flowedthrough effluent line 10 and into collection and distribution pipe 9.The collection and distribution pipe 9 is a hollow pipe preferablyperforated with a number of holes, preferably two rows of six holeseach, facing the bottom of the filter. One end of the collection anddistribution pipe 9 is connected to the effluent line 10 and the otherend, if the collection and distribution pipe 9 is perforated, is closed.The diameter of the holes is preferably about 3/8 inch. The holes causethe backwash water to be distributed uniformly into the filtration bed,making the backwash process more efficient. During normal filteroperation, the holes also provide an even distribution of effluent andhelp maintain effluent flow rate control. The backwash velocity employedin the preferred embodiment of the present invention is between 40 and65 gal./min./ft.². The backwash water passes through bottom screen 8,garnet layer 7, sand layer 6, granular plastic particle layer 5 and topscreen 4. The filtration media is fluidized between two screens. A planview of such a screen is shown in FIG. 5. The bottom screen 8 may bemade from stainless steel, plastic, or any other rigid, non-corrodiblematerial. Alternatively, the bottom screen 8 may be replaced by anyother porous support means for the filtration media, particularly inlarger filters. An example of such an alternative support means is afiltration brick. The particular filter brick used must allow backwashwater to pass through during the backwash process.

The fluidization rate of the plastic media is between 50 and 100percent, which creates a high backwash velocity to wash out the sludgecake and other accumulated sludge present in the filtration bed in ashort time. The fluidization rate of the optional garnet layer is low.This layer, which, as previously noted, may include garnet particles oftwo different sizes, is employed for the purpose of maintaining properbackwash water distribution.

Backwash frequency is determined by suspended solids concentration inthe influent liquid and by the volume of backwash water. In conventionalfilter operation, backwash velocity is between 20 and 30 gal./ft.² /min.for a dual-media (anthracite and sand) or a tri-media (anthracite, sandand garnet) filter. The backwash velocity employed in the presentinvention is between 40 and 65 gal./ft.² /min., which is about 100percent stronger than the conventional velocity. The wear-out rates ofthe plastic particles, sand, and garnet are low under high backwashvelocity using the filter of the present invention allowing the higherrate to be used. The strong backwash current shearing force created byhigh backwash velocity washes the sludge cake and other accumulatedsludge out of the filter vessel in a short time. The sludge in thefilter vessel 3 is pushed to the top by a plug flow adjusted by thecollection and distribution pipe 9, bottom screen 8 and garnet layer 7.Most of the sludge in the filter vessel 3 is pushed out of thefiltration bed at the beginning of the backwash step. Because of highbackwash velocity, only a short backwash time is required. The entirebackwash period takes only between 0.8 and 3 minutes, which is muchshorter than the time necessary for backwash in conventional filtrationsystems. The backwash frequency described earlier must be determined byevaluating the influent suspended solids concentration and hydraulicloading in the settling tank. The actual backwash frequency chosendepends on the desired effluent quality, head loss, and other individualcircumstances and is not a result of the configuration of the filter ofthe present invention. In the present invention, the backwash frequencyis controlled automatically by cycle timer 16 or by a float switch.

In a conventional filter system, where at least two filter media areutilized, backwash time usually takes between 5 and 10 minutes. In suchmulti-media filters, the backwash effluent will not cause hydraulicshock loading in the treatment plant. In home treatment plants and smallcommercial treatment plants, however, only one filter is usuallyemployed in the treatment system. A backwash of long duration in thecase of home treatment plants will cause hydraulic shock loading, whichaffects the overall treatment efficiencies of the system. In the presentinvention, a short backwash time, more frequent backwashing, and higherbackwash velocity are used to prevent hydraulic shock loading and sludgeclogging in the filtration bed.

Once the optimum backwash frequency is decided, the filter of thepresent invention can be automatically operated for a long period oftime. For example, a filter constructed according to this invention hasbeen tested in the test field of NSF International for over six months.The performance of the filter is satisfactory and will be discussed inexamples later.

It was stated earlier herein that the fluidization rate of the granularplastic media is between 50 and 100 percent. This high fluidization ratepushes the sludge particles out of the filter vessel in a short time,usually in 5 to 10 seconds. Because the structure of the plastic mediais much stronger than that of the anthracite particles used intraditional dual-media filters, the plastic media will last a longperiod of time in the filter of the present invention without anyplastic particle loss. During the backwashing process, plastic particlesare worn out very slowly even by violent mechanical rubbing. The plasticparticles always keep their granular form and will not be broken intosmall pieces as anthracite particles are under similar conditions.Although anti-electrostatic plastic media is more expensive thananthracite filtration media, considering media life and loss problems,the average daily cost of plastic media may actually be less than thecost of anthracite media. Also, the anthracite media cannot be operatedat all under high fluidization rate conditions, otherwise the media willbe totally lost very quickly.

The sand employed in the filter of this invention preferably has alarger diameter (on the order of 2 to 4 mm) than the sand used inconventional dual-media and multi-media filters (0.4 to 0.8 mm). Thefluidization rate of sand during backwashing is between 10 and 20percent. No sand loss problem has ever occurred during research testingof the present invention.

It is stated earlier herein that backwashing frequency can be controlledby a cycle timer or float switch.. If a cycle timer is employed in thefilter system, the backwashes must be set at low hydraulic loadingperiods, such as after 9:00 a.m., after 2:00 p.m. and after 8:00 p.m. Ifa float switch is used in the system, the goal filtration head loss mustbe determined before the installation of the filter system. The headloss or filtration head may be determined according to the standarddesign values for a treatment plant or by referring to other availablefilter drop values. Using a float switch, the backwash is controlled bya predetermined top flow line at which the filtration process will beterminated and the backwash process will begin. A disadvantage of floatswitch controlled backwashing is that the backwash may commence at anytime that the flow line raises up to the top level. If the backwashsteps are started during a peak flow period, hydraulic shock loadingwill be caused by adding backwash effluent into the treatment system.

The advantages of the present invention are illustrated in the followingexample:

EXAMPLE

A performance evaluation of filters constructed according to the presentinvention was carried out at the Michigan test site of NSFInternational. Tests of a tri-media filter, filter I (FIG. 3), and adual media filter, filter II (FIG. 6) were conducted in two wastewatertreatment plants by NSF agents. Filter II is the same as filter I, minusthe optional garnet layer 7. Sand was layered to a greater depth infilter II than it was in filter I, in order to compensate for themissing garnet filtering capability.

Filter I was installed to treat effluent from an activated sludgetreatment plant and filter II to treat effluent from a submerged biofilmtreatment plant. The configuration of these two filters is shown inTable 1. The operating conditions of both filters are listed in Table 2.

                                      TABLE 1                                     __________________________________________________________________________    Configuration of the Filters                                                                   GRANULAR                                                                      PLASTIC    SAND     GARNET                                   FILTER                                                                              HEIGHT                                                                              DIA. SIZE  DEPTH                                                                              SIZE                                                                              DEPTH                                                                              SIZE                                                                              DEPTH                                NUMBER                                                                              (Inches)                                                                            (Inches)                                                                           (mm)  (Inches)                                                                           (mm)                                                                              (Inches)                                                                           (mm)                                                                              (Inches)                             __________________________________________________________________________    I     28    8    2 × 3 × 3                                                               5    1.2-2                                                                             3    1.5-2.5                                                                           1.5                                                   cubic               2.5-3.5                                                                           1.5                                  II    24    8    D = 3.25                                                                            5      2-4                                                                             6    N/A N/A                                                   L = 3.0                                                                       pellet                                                       __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Operating Conditions of the Filters                                                          FILTRATION                                                                    VELOCITY                       MAXIMUM                               DAILY                                                                              PEAK                                                                              AT      BACKWASH                                                                              BACKWASH       FILTRATION                      FILTER                                                                              FLOW FLOW                                                                              PEAK FLOW                                                                             VELOCITY                                                                              TIME    CONTROL                                                                              HEAD                            NUMBER                                                                              (GPD)                                                                              (GPH)                                                                             (gal./min./ft..sup.2)                                                                 (gal./min./ft..sup.2)                                                                 (min.)  METHOD (inches)                        __________________________________________________________________________    I     500  100 4.8     43.0    1       Timer  24                              II    500  100 4.8     51.6    0.83    Timer  22                              __________________________________________________________________________

The volume of backwash water was 15 gallons per backwash. The backwashwater was returned to the primary settling tank through backwasheffluent line 15.

Summary performance data for filters I and II are shown below in Table3. The discharge requirement of the effluent are 30 mg/liter maximum forboth BODs and SS (Class I discharge standard).

                                      TABLE 3                                     __________________________________________________________________________    Summary Data of the Filter Test                                               FILTER                                                                              S.S. (mg/l)            BOD.sub.5 (mg/l)                                 NUMBER                                                                              INFLUENT                                                                             EFFLUENT                                                                             REMOVAL (%)                                                                            INFLUENT                                                                             EFFLUENT                                                                             REMOVAL (%)                        __________________________________________________________________________    I     42     13     69.0     39     25     35.9                               II    12.5   8.8    29.6     14.1   9.9    29.8                               __________________________________________________________________________

The summary data of influent and effluent using filter I show that theClass I effluent quality can be achieved by filtration using the presentinvention. Although the filter influent of filter II had alreadyachieved Class I requirements, 29.8 percent of BOD and 29.6 percent ofSS were removed using the dual-media filter of the present invention.Thus, an even better effluent quality was obtained. Filter I and II weresuccessfully tested for more than six months without any maintenance ormanual operation.

The dimensions of the present invention may be changed to suit aparticular application. For example, the size of the filter vessel 3 maybe enlarged to any size to accommodate industrial wastewater recyclingor for use in potable water applications. The size of the filtrationmedia used can also be adjusted to achieve certified effluent qualitybased on the influent quality and the desired discharge standards.

Preferred and alternate embodiments of the present invention have nowbeen described in detail. It is to be noted, however, that thisdescription is merely illustrative of the principles underlying theinventive concept. It is therefore contemplated that variousmodifications of the disclosed embodiments will, without departing fromthe spirit and scope of the present invention, be apparent to personsskilled in the art.

What is claimed is:
 1. A filtration system for the removal of suspendedsolids and biochemical oxygen demand from a fluid, comprising:(a) afirst container having a hollow inside area; (b) an influent line havingan interface with the first container and providing a passage for fluidto the inside area of the first container from outside the firstcontainer and further having a check valve for preventing the flow offluid through the influent line from the inside area of the firstcontainer to outside the first container; (c) a first effluent linehaving an inner end, an outer end, a hollow inside surface, and anoutside surface and further having an interface with the first containerat the inner end of the first effluent line, the first effluent lineproviding a passage for fluid from the inside area of the firstcontainer to outside the first container; (d) filtration media, locatedinside the first container, the filtration media comprising:(i)anti-electrostatic glass-filled nylon granules having a specificgravity; (ii) sand particles having a specific gravity; and (iii) garnetparticles having a specific gravity; (iv) all the glass-filled nylongranules being larger in dimension than any of the sand particles, thespecific gravity of the sand particles being greater than the specificgravity of the glass-filled nylon granules, and the specific gravity ofthe garnet particles being greater than the specific gravity of the sandparticles; and (v) wherein the glass-filled nylon granules are of asubstantially uniform size; (e) a top screen located in the inside areaof the first container between the influent line and the filtrationmedia; and (f) a filtration media support located inside the firstcontainer between the filtration media and the first effluent line. 2.The filtration system of claim 1, wherein the glass-filled nylongranules have a specific gravity of about 1.3.
 3. The filtration systemof claim 1, wherein the size of the glass-filled nylon granules rangesfrom about 2 millimeters to about 4 millimeters.
 4. The filtrationsystem of claim 1, wherein the size of the sand particles ranges fromabout 1.2 millimeters to about 3.5 millimeters.
 5. The filtration systemof claim 1, wherein the garnet particles comprise a first group ofgarnet particles having a first size and a second group of garnetparticles having a second size, the second size being larger indimension than the first size.
 6. The filtration system of claim 5,wherein the first size of the garnet particles ranges from about 1.5millimeters to about 2.5 millimeters and the second size of the garnetparticles ranges from about 2.5 millimeters to about 3.5 millimeters. 7.The filtration system of claim 1, wherein the filtration media supportis a bottom screen.
 8. The filtration system of claim 1, wherein thefiltration media support is a filtration brick.
 9. The filtration systemof claim 1, wherein the interface between the first effluent line andthe first container is a hollow pipe having a first end and a secondend, located within the first container, the pipe being connected to thefirst effluent line at the first end.
 10. The filtration system of claim9, wherein the hollow pipe is closed at the second end and is perforatedwith a plurality of holes.
 11. The filtration system of claim 1, furthercomprising a backwash pump adapted to send fluid into the firstcontainer through the first effluent line when operated, the backwashpump being connected to the first effluent line at the outer end of thefirst effluent line;the filtration system further comprising a secondeffluent line having an interface with the first container and providinga passage for fluid from the inside area of the first container tooutside the first container, the top screen being located between thesecond effluent line and the filtration media; the first effluent linefurther comprising a fluid passage disposed to allow the passing offluid between the hollow inside surface of the first effluent line andthe outside surface of the first effluent line; the backwash pump beinglocated within a second container having a hollow inside area and anoutlet port; the filtration system further comprising an automaticcontroller adapted to actuate the backwash pump; and the automaticcontroller being selected from the group of controllers consisting ofcycle timers and float switches.
 12. The filtration system of claim 11,wherein the first container and the first effluent line are locatedwithin the second container.
 13. The filtration system of claim 1,wherein the filtration media further comprises activated sludgeparticles.
 14. The filtration system of claim 13, wherein the activatedsludge particles are in the form a cake at the top of theanti-electrostatic glass-filled nylon granules.
 15. A filtration systemfor the removal of suspended solids and biochemical oxygen demand from afluid, comprising:(a) a first container having a hollow inside area; (b)an influent line having an interface with the first container andproviding a passage for fluid to the inside area of the first containerfrom outside the first container; (c) a first effluent line having aninner end, an outer end, a hollow inside surface, and an outside surfaceand further having an interface with the first container at the innerend of the first effluent line, the first effluent line providing apassage for fluid from the inside area of the first container to outsidethe first container; (d) filtration media, located inside the firstcontainer; (e) a top screen located in the inside area of the firstcontainer between the influent line and the filtration media; and (f) afiltration media support located inside the first container between thefiltration media and the first effluent line; (g) the filtration mediacomprising:(i) anti-electrostatic glass-filled nylon granules having aspecific gravity; (ii) sand particles having a specific gravity; and(iii) activated sludge particles in the form of a cake at the top of theglass-filled nylon granules; (h) the glass-filled nylon granules beinglarger in dimension than the sand particles; and (i) the specificgravity of the sand particles being greater than the specific gravity ofthe glass-filled nylon granules.
 16. The filtration system of claim 15,wherein the filter media further comprises garnet particles having aspecific gravity that is greater than the specific gravity of the sandparticles.
 17. A filtration system for the removal of suspended solidsand biochemical oxygen demand from a fluid, comprising:(a) a firstcontainer having a hollow inside area; (b) an influent line having aninterface with the first container and providing a passage for fluid tothe inside area of the first container from outside the first container;(c) a first effluent line having an inner end, an outer end, a hollowinside surface, and an outside surface and further having an interfacewith the first container at the inner end of the first effluent line,the first effluent line providing a passage for fluid from the insidearea of the first container to outside the first container; (d)filtration media, located inside the first container; (e) a top screenlocated in the inside area of the first container between the influentline and the filtration media; and (f) a filtration media supportlocated inside the first container between the filtration media and thefirst effluent line; (g) the filtration media comprising:(i)anti-electrostatic glass-filled nylon granules having a specificgravity; (ii) sand particles having a specific gravity greater than thespecific gravity of the glass-filled nylon granules, the sand particlesfurther being smaller in dimension than the glass-filled nylon granules;and (iii) garnet particles having a specific gravity greater than thespecific gravity of the sand particles, the garnet particles furtherranging in size from about 1.5 millimeters to about 3.5 millimeters. 18.The filtration system of claim 17, wherein the glass-filled nylongranules are of a substantially uniform size.
 19. The filtration systemof claim 17, wherein the filter media further comprise activated sludgeparticles in the form of a cake at the top of the glass-filled nylongranules.